Carrier telegraph circuits



Ja-n. 18, 1927.

. 1,614,545 R. w. DEARDORFF CARRIER TELEGRAPH CIRCUITS Filed July 31, 1923 2 sheets-sheet' 1 1,614,545 Jan 18 1927 R. w, DEARDORFF CARRIER TELEGRAPH CIRCUITS Patented Jan. 18, 1927.

1,614,545?. rare-Nr RALPH W. nnAn-nomar,'onrnooxirn;` nnurafomi. ASSIGNOR' 'ro AMERICANTEIZ'E- PHONE" AND TEnneRArHCOMrANY, A CORPORATION '0F NEW YORK? CARRIER TELEG'RAPI` CRCU ITS.

Application le'd July 31,

This invention relates to multiplex signaling," and'imore lparticularly to multiplex systems einployiiig"cariier currents 'forthctransmission of telegraph signals.

the objects of the invention-is to provide a inultipler'carrier telegraph circuit arrangement inwhich the' 'circuit apparatus may bedvided at' suchfa point 'in 'the circuit that' the'loiv frequency apparatus lcan `be sepa rateclffrom the high frequency apparatus, thus permitting' of improved vni'ethods of mounting 'the 'equipment unitsl upon racks and also-enabling thesoperationo east and west repeatinglcircuits on adjacentisets. Another objectv of "theinvention is to provide `a multiplex' carrier telegraph circuit which will permitlof vacuuintube repeating betweencarrierline sections at loW requen-cy and Without thej'interposition' ot relays' between 'the receiving Achannell oi the* incoming' line andthev sendiligchannel of' the outgoing line: Another object or the invention is to provide a circuit arraiigement'of the class? above described` which will more effectively `exclude beatrequencies from thev circuitor' the receiving frelay of the lrecei-v-` ing 'channel'. the* 'invention vwill more fully appear herein after.

In multiplex carrier fact" necessary toarrange upon a common panel all of' "the" apparatus 4of a number olil channelssuperimposed upona given line.`

units may' thenA be'symmetrically mounted i upon racks, landflvvh'ere *(as Will usually be` the-case)y fit Vis Vnecessary Ytolocate the 'low 'l'reqi'iencylapparatus `at a different point of l the'i o'llifc'e' *from the high frequency appara-1k One of" Other and further objects off telegraph systems o'lf t-heprio'rA art, ithas been'custoniary and in TheA 'locatioirofthe apparatus upon* the panel' was determined by the amount ot be made i'tllroughfthei office f cabling. With" 1925. Serial No. 654,943.

the apparatus segregated'I and Vinounteldfiiir this manner, it becomes a very' simple mat-- ter to patchlany given unitolfl'ovif'requency" apparatus toany desired unitilofhigh- -ne quency apparatus so i that 'the -arrangementfo becomes much more 'flexibler Another diiiiculty encountered in-connec tioir with prior art systems has todolfwitli l thel interconnection ofi incoming4 channels of one linevvith outgoing channels ofanother 65 linefor through repeating; It liasheretoM tore been1 impossible `v toy patchvv the carrier" channelsof" oppositedirectionsto adjacent panels because :of the excessive'lo'sses Wl'i'ichf' are introduced into the high frequency cir-#10 cuit by the otiice cabling.- It is apparent,-`- lioivever, that it' the high frequency signals arefconverted to loiv frequency*signalsjbe-` tore the patch is made, it Will' be l"possible toliitroduc'e a considerable amountofofice cabling Without materially ailielctiigf theL quality `of the telegraph signals.

ln order to attain thisfresult and-the other results discussed, the arrangement of the;

vacuum tubes inthe receivingu channel'isSO Y changed. I-leretoforeA the first tubefor tubes inthe circuit have been used as-high "ifre':

quency ampliiers and the flasttubeasfa=rec=` titier ordetector Afrom the output of= which the receiving relay is operated. In'aceordef ance With.- the present invention,` the-A first*- tub@ will! servefasa detector andA 'theL sulese` quent tubes as pliliers.`

This renders it possible to keep suehequipment'as the amplifiers and receiving relays avi/ay-4A ifroin the racks upon Which the higl-'il'fr'e-'- quency units are mounted, in order that adA` justinents which are necessary-Ito 'compensate 1I direct current tor signal amforfiveather variations may -beinadef-by thef rea'rrangen'ient,V off the apparatus-l0 The rearrangement of the circuit above referred to, together with the distribution of the apparatus in the office racks, renders it possible to patch from the outputV of the detector to the modulator of the transmitting channel, so that signals may be repeated in the low frequency state directly from the detector of one receiving channel to the modulator ef any given transmitting channel without the use of intervening re lays. rlhe patching may be done at a point near the racks or tables upon which the low frequency equipment is mounted by carrying leads from the output of the detectors and from the input of the modulators to these points through the office cabling. frs the transmission through the cabling ill thus take place at the low frequency state, no difficulty will be involved in patching from one circuit to another as the exigencies of a particular situation may warrant.

By rearranging th apparatus of the re ceiving channel so that amplification takes place after detection, the filter, which will be inserted between the detector and amplifier to suppress frequencies higher than the low frequency signals which are to be received, may readily be designed to cut off any interfering beat frequencies higher than the normal telegraph frequency.

The invention may now be more fully understood from the following detailed de* scription when read in connection with the accompanying drawing in which Figure l is a simplified diagram showing the arrangement of the apparatus in a receiving channel in accordance with the present invention. Figs. 2, 3, and 5 are curves illustrating the operation of the different vacuum tube elements of the receiving channel. Fig. 6 is a simplified diagram showing the circuit arrangements of a typical transmitting channel and a typical receiving channeh'to gether with patching arrangements for patching between different high frequency and low frequency units and for patching between transmitting and receiving channels for through repeating. Fig. .7 is a curve illustrating the operation of the modulator in accordance with the present invention. Fig. 8 is a diagram similar to that of Fig. G, showing a modified arrangement, and Fig. 9 is a circuit diagram showing how the Vunits of the channels may be interconnected through the oflice cabling.

As has been previously stated, the principle involved in the present invention is that ofl detecting in the first tube of the receiving circuitl and the use of direct current amplification in the succeeding stage or stages. This is illustrated in simplified form in l in which RF is a tuned circuit or band filter interposed between the receiving channel and the high frequency transmission line to select te the channel the particular band of modulated carrier frequencies which carry the signal for that particular channel. A detector D, such as a vacuum tube detector, for example, is connected in circuit on the channel side of the lter. A low frequency amplifier A., which is preferably of the vacuum tube type, is included in circuit beyond the detector for amplifying the detected low frequency signals. The receivling relay RR is connected to the output circuit of the amplifier it., and a low-pass filter RF is included between the detector D and the amplifier A.

The input circuit of the detector D is provided with a grid leal: l0 and a condenser l1, these elements being of a well known type. This arrangement of the input circuit has certain advantages over the so-called C battery type of detector, as it is found that in telegraph work a slightly larger amount of gain can be realized in the detector tube. An additional advantage of this circuit re- .sides in the fact that the phase of the received signals will come out right with. a smaller number of tubes when the grid leakcondenser arrangement is used than when the C battery type of detector is used.

rlhis will be clear from the curves of Figs. 2, 3, 4 and 5. Fig. 2 is a curve showing the changes of the grid potential with time. Normally the potential of the grid will have a certain positive value as indicated, this positive value being due to the connection existing between the grid and the battery for heating the filament. As carrier oscillations are impressed upon the grid, however, the succeeding oscillatons are trapped upon the grid by means of the condenser (due to the fact that current can only flow one way through the tube) until a state is reached dependent upon the rate of leak through the resistance l0 such that no further increase in the negative charge trapped upon the grid can take place. The potential will then vary in accordance with the high frequency waves, but the average potential will remain fairly constant. At the end of the train of waves the potential will again assume its normal value, the negative charge on the grid having leaked away through the resistance 10.

Fig. 3 indicates the change in the output current of the detector tube D. The current will have the normal positive value determined by the normal positive potential of the grid, but as the potential of the grid decreases or changes from positive over toward the negative assuccessive charges are trapped upon the grid in response to the received waves, the output current decreases. At the end of a train of waves the output current again assumes its normal. value. It will thus be seen that a train of waves, corresponding to a telegraph dot or dash, produces during the interval that the train persists any average -reduction inthe potential appliedto-y the-detector tubeand al corresponding.; average reduction; ini the` outputl current.

w At the amplifier A, however, theA phase of-4 the; :low frequency 'wave is reversed. Non mallya potential is appliedto the gridof the'amplier whichfhas` a negativevalue (determined by the: C battery andby. ther ==plate=potential fromv the detector tube' D),

such that no current fiowsin'the output cir-- cuitJas-will be clear from a; comparisonofA Figs. 4 and 5. When, however, a decrease inotherwor.ds,.-its potential is raised. This,` produces-fan increase inthel positive corn-I ponent of the potentialy applied to thegrid.-

V'of amplifier tube A, sol that the potential of the gridi of the-'tube A shifts from a givennegative value :toward they positive as indicated:i by the -curveofF-igd. As ythe grid becomes: less negative, thel plate current ina5creasesrasindicated in Figsz.- The current-- curve Aof Fig. 5 is properly phased tooperate thereceiver ifa current pulse-or an increase.r incurrentvcorresponds to vatrain of carrier oscillations .impressed upon the line. 4010 amplifier l'A hasupractically infinite impedance due to the fact that no current can flowy bet-weenfthe grid andthe filament whenthe 451 grid is made sufficiently negative, the impedance,- looking, int-o the filter from the detector,

will not'be; equivalent to thatof an infinitely` long-line. It isl therefore desirableto termi natej the filter on the sideadj acent the ampli-- fier in a simple network consisting. of yresistancesf 14; land- 15 Vandcapacity, 16 designed:

andfproportioned' in a well known manner to simulatej theV impedanceof an infinitely longwline.` over the range of frequencies'j passed bythe filter. The provision of this networlrgenables the filter to give a close aprv proximation'fof the action of a theoreticI filter having an infinite numberl of sections. The filtery RF, which isf provided primarily ""inorderto keepthe highfrequencies which appear in the output circuit of the'detecto'rx' from being transmitted to the input circuit ofthleamplifier, also maybe made to serve anfadditional purpose; viz., that of'suppress- 5W' ing' beat v.note Afrequencies of` the order of The filter-IRF isV composed of relatively few sections- As the input circuit of thev 200- or] 300,cycles; fwhiclr sometimes' f result from; the. interaction in the detector of` in.4 terferinge frequencies. f

In applyingA va 'circuit of.` the f-type illus? trated inlfigyl to plant operation, it is de-v sirable to utilizej the-120s volt. Morse bat-A teries as well as the 24 Voltbattery Icom-l monly "usedinplants for the purposeof sup-i plying; the plate` and filament currents andi.`

also' theV G .or grid potential. Thesebatteries;y

are normally, groundedA asvused 1 in; the plant-,fv

and' whilejthis does not' interfere .with their use for filament and plate currents, as: Will- -be apparent from-the dotted. ground connec-y tions indicated-in Fig. 1, it is 'impossible-tos use a grounded battery 'fOr-supplying lthe grid potential of-theamplifier It. willflben apparent from the diagram Y that. if Kground be appliedon either sideof the Cbattery` o f. the` amplifier A, it willfresult; infshort` circuiting the. grid, sothat thecsignalfposf tentialswill noty be appliedto thel grid and; hence will not bel amplified Thiswvculd= necessitate v the provision. of a separate dryicell= batter-y for each amplifier:

Fig., 6- illustrates a` circuitf arrangement inF whichy this.- diiiicu-lty. is overcome. Fig., 6'

also illustrates the mannerin which 3the high frequency andr low frequency app aratusv may be segregated .inlto.- unitsf and the various patching Iconnections effected. A transmit-- ing; channel and. receiving` :channel are illustrated ,in some detail' in Figa, andwtheir manner ofT association with carrier Vfrequency lineslgis also indicated;v Ther-transmitting; and receiving-.channel,.shown indetail, arey not, however, arranged. to be operated-toa gether in connection with-onecarrierf line# to form a two-Way communication channel, but` are-eachshown as connected todierent linef sect-ions incomngtoythe station from different directions, sothat the-twofchannels may be,-if desired,.connected together torepeat fromzthe oneli'ne sectionhtothe otlier.l Accordingly, the line LW, going west, is shown. .terminated in bus bars to which are con-y nected a pluralityof transmittingfand receiving channels separated from eachother by` filters such` as` RF; The-details of the` transmittingA` channels associated with the line LW are not shown, it being understoodVA that Vthese channels' will be similar to the channels illustrated in connection with theline LE to fbe described hereinafter. The?l apparatus associated w1thone receiving channel only is lshown in detail and comV prises the receivingdilter-RF, aidetectorfD, I

a low-pass-filter'RF and A. DC. amplifier A,` having A,the receiving relay RR in its4 out-- put circuit.

preventinggthe 'transmission of higl'r frequencies. itc the receiving f relay; RR."

If desired, the output circuit of L the amplier may include a coil or filter, foi" The* detector D is of: -the-grid leakltypef:

illustrated in Fig-1.r The low-passcfilt-er Cil RF is also similar to that of Fig. 1, but in this instance the network 14, 15 and 16 is omitted, and the filter is terminated by a resistance 18 which is made substantially equal to kthe impedance of an infinitely long line at the principal frequency to be transmitted through the filter.

The filaments of the detector D and amplifier A are supplied with current from the grounded 24 volt battery. The plate currents are supplied from the positive side of the grounded 120 volt Morse battery. In order to provide the necessary C potential for the grid of the ampliiier A from the negative side of the 120 volt grounded Morse battery, the positive and negative Morse batteries are connected to a resistance bridge comprising resistances 18, 19 and 20. It will be apparent that there will be a continuous change in potential through the several resistances beginning with 120 volts positive at the lower terminal of resistance 18 passing through a neutral point of Zero potential at 21 of resistance 19 and ending in a point Whose potential is 120 volts negative at the lower terminal of resistance 20. The plate of the detector D is connected to a point 22 so chosen that the resistance 18 will have the desired value for terminating the filter RF and so that the point ivill have the desired positive potential for the plate. A point 23 is also chosen having the desired negative potential for the grid of the amplifier A. A suitable potentiometer may, if desired, be included in the input circuit of the amplifier A for the purpose of making minor adjustments in the C potential.

Jacks 25 and 26 are included in circuit between the detector D and the amplifier A, as indicated. These jacks may be positioned at a testboard, or at some point adjacent to the receiving operator, thereby enabling the attendant to sivitch the low frequeny apparatus, which is preferably mounted on a table or rack at or near the operating room, into connection with any desired detector and associated apparatus of any line.V This permits of great flexibilityV of operation. The detector and associated high frequency equipment may be mounted as a unit upon a rack at some distant point in the oiiice.

LE designates another high frequency or carrier line extending east from the sta-tion illustrated, said line tern'iinating in bus bars to which are connected a number of transmitting and receiving channels. The apparatus of the receiving channels is not illustrated but Will be understood to be similar to that described in connection With the line LlV. The apparatus of only one transmitting channel is-illustrated and includes a tuned circuit or band filter TF for separating the channel from the other channels associated With the same line. A modulator is associated With the filter, the modulator comprising a. vacuum tube arrangement M Whose input circuit may be normally supplied With a carrier frequency from a circuit 27 leading to some suitable carrier source such as, for example, a vacuum tube oscillator. The frequency supplied to the modulator from the oscillator may be interrupted in accordance with signals by a connection 28 leading from the grid through a low-pass filter TF to a transmitting apparatus TR. The iilter TF serves the purpose of curbing or rounding off the telegraph or Morse impulses.

Ja'cks 29 and 30 may be included in the circuit 28 at a testboard or other point near the operating table whereby the transmitter may be patched to any desired modulator. This arrangement enables the lou7 frequency transmitting apparatus to be separated from the high frequency apparatus, such as the modulator and its associated equipment. The high frequency equipment Will preferably be made up as a unit and mounted on racks along with the high frequency units of the arious receiving channels. By means of the jack 29 in the transmitting circuit and a jack 31 associated with the output of the detector D. a connection may `be established through the patching cord 32, so that the detector D repeats directly into the modulater M. `In this manner any receiving channel may be patched to any transmit-ting channel for through repeating, and it will be observed that the signals are repeated in their low frequency state Without the use of any relays. This is advantageous Where the repeat-ing connection passes through a considerable amount of office cabling, as the attenuation of the cable does not seriously distort the low frequency signals, Whereas, if the transmission took place at carrier frequencies, a much greater amount of distortion would occur.

lilith this method of repeating some provision must be made to keep the amplitude of the signals fairly uniform under different attenuation conditions of the circuit due to Weather varia-tions, as otherwise the effects of the Weather variations Would have a cumulative action in a system having any considerable number of repeater points. For example, if the condition should be such that the incoming detected signal obtained from the detector of one repeater is greater than the normal signal, it will be impressedvupon the modulator of the corresponding repeater to produce a greater amplitude of variation at the transmitting point than will ordinarily be sent out, Accordingly, at the next repeater point the signal Will be still greater, and it may finally become so great as to overload the tubes.

This diiiiculty may readily be remedied by operating the modulating tubes at saturation. The action of t-hese modulators Aplied to thefirst amplifier A, so 'that a cormay be understood fromFigf, which shows as the saturation partv of the curve tends to maintain 4.the amplitude constant, and the only effect ofthe increase in amplitude of the input potential will be to produce a fiattening of the tops of the waves in the outi put circuit.

Where a suflicient amount of amplification ish-not obtained in the receiving channel by the use of asingle amplifying tube, it may be desirable to use a detector of the. C batterytype, as this type of detector will have the effect .of bringing-out the received wave Lin the output of the additional amplifying tube in the right, phase. A circuit for this purpose is illustrated in Fig. 8. In this figure, A represents the detecting tube and A and A two low frequency amplifying tubes.

-The de ctecting tube is supplied with a C Ybattery potential in any well known manner,

while'theplate potential of the detector and the CV potential of the amplifier A are supplied by a resistance bridge in the manner described in connection with Fig. 6. u

plate 'potentialof the amplifier A and grid The "potential of `the amplifier A are likewise ed so that the detector is operating near thel providedthrough asimilar resist-ance bridge arrangement.

The effectvof a train of carrier oscillations corresponding to a signal when impressed upon the grid of the detertor D is to produce an increase in the current fiowing in `the plate circuit of the detector, as-tlie normal C Vpotentialof the grid will be adjustlower linee of its characteristic curve.

il/Vlien operated at this point, the effect of the positive half of each high frequency wave is tofproduce an increase inthe normalplate current, l while the effect of each negative hialfwave is to produce a much smaller de- `crease inthe normal plate current. ltrain of wavesv therefore produces on the Aaverage an` increase inthe plate current. Ac-

The

cordingly, if we contrast thisy action with the action indicated by the curves of Figs. 2,E {3,4 and 5, it will be seen'that the in-` crease .in Athe current Vin the `detector cirresponding decrease in current will appear cuitf'vThis produces a wave in the output :circuit ofithe amplifier A which corresponds tozlthel .wave for'` the tube- A of Fig. 1, as il- Vphase relation.

lustrated in Fig. 5. iConsequently@the coin- Ybination is such that the wave inthenutthe amplifier A for adjustingthegain. l'-The second-stage amplifier, whichA :is provided with a potentiometer, will' then be mounted withthe relay equipment at a point near the operators station. rilie jaclrl is .connected ,to .the output circuit of the amplifier rA in Fig. 8 rather than tothe output circuit of the ,detector Das in the case of'Fig. 6, for the reason that, when a `patching.connection isto be established through jaclrsil and29 to a transmitting channel for through repeating, the low frequency wavev will haveVV` they right phase for this purposeA as it appears in the outputcircuitlof tliefirst amplifier. This is dueto the reversal of phase introducedfby the C battery type of detector. This circuit arrangement has .the same advantages from Athe standpoint of mounting the apparatus 'in units and 4from the standpoint of office cabling as that ofv F ig. .6.

A circuit is illustrated in Fig..9 somewhat y similar to that of Fig. 6, this circuit being arranged toshow how the cabling` between the high frequency apparatus mounted on Yone set of racks and thev low frequency units .mounted upon other racks may be accomput of the amplifier A, thusinaking thevlow-i H5 passffilter apart of the low frequency equipment. AThis cabling then intervenes between Ithe low-pass filter and the output sidel fjthe detector. This prevents the' grid `of thedirect .current amplifier A from picking 1upM interfering potentials which `are usually present `in office wiring. :The resistance bridge v18, 19 and `20, `through which the plate potential of the detector D andthe K grid potential of the amplifier AVareHpro-*u vided, isalso made a part of the'y low` frequency equipment. `A filtering 'arrangement 35 is provided in the negativelMorse battery tap which supplies the grid potential for the purpose of reducingffiuctuations" lIO and noise which are usually present in such batteries. The patching jacks for patching the low frequency equipment to any detector unit are arranged between the filter and the detector at a testboard near the rcceiving operator, so that the patching may be readily accomplished.

The transmitting equipment is substantially the same as that illustrated in Figs. G and 8 except that the connection Q8 from the grid of the modulator to the transmitter TR includes a 120 volt C battery at the transmitter. This C battery enables the modulation to be accomplished by varying the grid potential rather than by short-circuiting the oscillator by grounding the grid as in the previous figures. By using C battery modulation, the transmitting relay TR. may be mounted at the operating table instead of at some point near the modulator as would be necessary if the ground method of modulation were employed. r1`his is for the reason that, if the ground were applied at some distant point involving a considerable length of cable in the wire 28. the effeet of applying the. ground would not be to short-circuit the. modulator but only to partially shunt it, so that as a consequence the modulation would be less effective. The

C battery method, however, enable the adjustment of the potential applied when the C battery is connected to the grid to such a Value that no current flows in the output circuit, thereby, in effect, completely interrupting the transmission of carrier oscillaorder to draw current through the inductance, the inductance 37 having no effect npon mere potential changes. The jacks 29 and 30 serve for patching between different transmitters and modulators as in the other figures, while jacks 31 and 38 may be used to patch between the output of the detector D and the input of the modulator M for through repeating. rlhe various patching jacks are all preferably arranged at the Morse board, or other convenient point, so that great flexibility in patching up connections is possible.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the scope of the invention as defined in the following claims.

That is claimed is:

l. In a carrier-telegraph system, a transmission line terminating at a central station, a receiving channel associated with said transmission line, said Channel including a detector for detecting low frequency telegraph signals from the carrier currents rcceived, said detector being located at one point in said station, cabling extending from said point to a distant point in said station,"

an amplifier for said channel functioning to amplify detected signals, said amplifier bcing located at said distant point at said lstation, and connections between said detector and said amplifier' extending through said office cabling.

2. In a carrier telegraph system, a plurality of carrier receiving channels each having high frequency apparatus including detectorsfor detecting telegraph signals from received carrier currents and each having low frequency apparatus including amplifiers for amplifying the received signals, and switching arrangements whereby the low frequency apparatus of any channel may be disconnected from its corresponding high frequency apparatus and connected to the high frequency apparatus of any other channel.

3. In a carrier telegraph system, a'plurality of carrier receiving channels each having high frequency apparatus including detectors for detecting telegraph signals from received carrier currents and each having low frequency apparatus including amplifiers for amplifying the received signals, and switching arrangements whereby the high frequency apparatus of any channel may be disconnected from its associated low frequency apparatus and connected to the low frequency apparatus of any other channel.

4t. In a carrier telegraph system, a carrier transmission line. a carrier receiving channel associated with said line, said channel including a high frequency filter for selecting the carrier frequency assigned to the channel, a detector for translating the selected carrier frequency into low frequency telegraph signals` a low frequency filter for selecting the detected low frequency signals to the exclusion of higher frequencies and a low frequency amplifier for amplifying the selected low frequency signals, said detector and high frequency'filter being segregated to form one unit and said low frequency filter and amplifier being segregated to form another unit.

5. In a carrier telegraph system, a carrier transmission line, a carrier receiving channel associatedwith said line, said channel including a high frequency filter for selecting the carrier frequency assigned to the channel, a detector for translating the selected carrier frequency into low frequency telegraph signals, a low frequency filter for selecting the detected low frequency signals to the exclusion of higher frequencies and a low frequency amplifier for amplifying the selected low frequency signals, said detector and high frequency filter being segregated to form one unit and said low frequency lilter and amplifier being segregated to form another unit, and oiice cabling intervening between the two units.

6. In a carrier telegraph system, an incoming receiving channel including a detector for detecting telegraph signals trom the received carrier currents and an ampli-- tier and amplifying the detected signals, an outgoing transmitting channel including means for supplying a carrier frequency and almodulator to which the carrier frequency is supplied, and means for patching a direct galvanic connection from the output circuit of the detector to the input circuit ot the modulator whereby detected low trequency signaling currents may be repeated directly to the modulator without intervenin(T relays.

In a carrier signaling system, a plurality of incoming receiving channels each including a detector for detecting low frequency telegraph signals from the carrier frequencies transmitted to the channel, and an amplifier for amplifying the detected telegraph signals, a plurality of transmitting channels each including a modulator, and means for supplying carrier currents to the modulator, and switching arrangements whereby a direct galvanic connection may be established from the output ot the detector of any receiving channel to the input olf the modulator' of any transmitting channel so that telegraph currents detected in that receiving channel may be repeated directly to the modulator ot the connected transmitting channel without intervening relays.

8. In a carrier signaling system, a carrier transmission line,` a receiving channel associated therewith, said channel including a detector for detecting` low frequency telegraph signals trom carrier currents transmitted over the line to the channel, said channel also including an amplifier for amplifying the detected signals together With a relay responding to the amplified signals,

' a second carrier transmission line having a transmitting channel associated therewith, said transmitting channel including a modulator, means torsupplying carrier oscillations to the modulator together with a telegraph transmitter or controlling the supply ot oscillations to the modulator', and means whereby a direct galvanic connection may be established from the output of the detector in the receiving channel to the input ot the modulator in the transmitting channel thereby establishing a circuit Whereby repeating may take place between said transmission line section Without intervening llorse relays.V

9. An amplifying arrangement comprising two vacuum tubes in tandem, a bridge having its terminals connected to opposite poles ot a battery whereby' an intermediate point ot' said bridge willbe neutral, a connection from the plate of the first of said tubes to a point on said bridge having a positive potential and a connection from the grid ot' the succeeding'tube to a point on said bridge having a negative potential.

l0. An amplifying arrangement comprising vacuum tubes in tandem, a bridge arangenient, a pair oi batteries having their opposite terminals connected to the terminals oit said bridge and their other terminals grounded whereby an intermediate point of said bridge will be neutral, a connection 'from the plate of the iirst ot said vacuum tubes to a point on said bridge having a positive potential and a connection from the grid ot the succeeding tube to a point on said bridge having a negative potential.

In testimony whereof, I have signed my name to this specification this th day of July, 1923.

RALPH W. DEARDORFF. 

